1. Field of the Invention
The present invention relates to a polarization-maintaining optical fiber.
2. Description of the Related Art
Conventionally, a polarization-maintaining optical fiber is used to, for example, connect optical devices each other having polarization dependence in an optical transmission system or the like.
As this kind of polarization-maintaining optical fiber, as disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. 2003-337238 (hereinafter, refer to Patent Document 1), a polarization-maintaining optical fiber is known which includes: a pair of stress-applying parts provided at both sides of a core; and a cladding surrounding the core and the paired stress-applying parts.
In recent years, there is a growing need for a reduction in size of a module or the like which includes the above-described optical device. In accordance with this, it is required to reduce a size (curvature radius) of a polarization-maintaining optical fiber used in such module and use it therefor.
In the aforementioned conventional polarization-maintaining optical fiber, as a result of setting a relative refractive index difference between a core and a cladding greater, it is possible to increase a degree of optical confinement into the core.
Accordingly, even where a polarization-maintaining optical fiber is bent so as to have a small radius, it is possible to reduce an increase in a bending loss.
Furthermore, in the aforementioned conventional polarization-maintaining optical fiber, even where the polarization-maintaining optical fiber is bent so as to have a small radius by arranging the paired stress-applying parts close to the core, it is possible to reduce an increase in polarization crosstalk (bending polarization crosstalk).
Moreover, conventionally, as other optical fibers which can reduce an increase in a bending loss or polarization crosstalk even where the optical fiber is bent so as to have a small radius, a polarization-maintaining photonic crystal fiber is also known in which a plurality of air holes around a core area are formed (Katsuhiro Takenaga, et. al, “Broadband Polarization-Maintaining Photonic Crystal Fiber”, Technical Report by Fujikura Ltd., April, 2005, 108th, p6-9, hereinafter, refer to Non-Patent Document 1).
However, in the polarization-maintaining optical fiber disclosed in Patent Document 1, there is a problem in that, as a relative refractive index difference between the core and the cladding increases, mode field diameter (mode field diameter) becomes smaller.
In this case, a difference in a mode field diameter between the above-mentioned polarization-maintaining optical fiber and an existing optical fiber optically connected thereto (single-mode fiber (SMF) or other polarization-maintaining optical fibers) increases and a connection loss between the above-mentioned polarization-maintaining optical fiber and the existing optical fiber thereby increases.
In addition, as the mode field diameter of the polarization-maintaining optical fiber becomes smaller, it is difficult to optically connect the polarization-maintaining optical fiber to the optical device.
On the other hand, in the case of using the polarization-maintaining photonic crystal fiber in disclosed in Non-Patent Document 1, it is possible to reduce an increase in a bending loss or polarization crosstalk when the optical fiber is bent so as to have a small radius while setting the mode field diameter large; however, it is difficult to manufacture the polarization-maintaining photonic crystal fiber and it is also difficult to reduce the manufacturing cost.